Northern California's kelp forests have disappeared since 2008

Warming oceans have shrunk kelp forests, which support marine ecosystems, by over 99% near Sonoma and Mendocino counties

By Ilena Peng

Northern California has lost over 99% of its kelp forests in just over a decade, according to research from a team at the University of California.

Using satellite imagery, the researchers detected over 3.5 million square meters of kelp in 2008 along the coastline bordering Mendocino and Sonoma counties. By 2019, that number dropped to fewer than 11,000 square meters.

California’s kelp forests, sometimes referred to as the Sequoias of the Sea, are the backbone of a healthy ocean ecosystem. The waving fronds sustain small critters like snails, crabs, sea urchins and abalone, which in turn feed otters and other larger animals. But as climate change has contributed to warming waters, the forest has started to disappear.

"Imagine you were walking in the forest on your favorite trail, surrounded by tall beautiful trees, singing birds, flowers, animals, etc," said Meredith McPherson, the lead author and a postdoctoral researcher at University of Massachusetts Boston.

"Then imagine that suddenly one day you went back for a hike and that forests had disappeared. Completely replaced by nothing other than dirt and maybe some weeds. Imagine that it stretches for hundreds and hundreds of miles. That is what has happened to the kelp forests in northern California."

Westport

Fort Bragg

Mendocino

MENDOCINO COUNTY

Anchor Bay

SONOMA

COUNTY

Stewarts Point

Fort Ross

25 km

Westport

Fort Bragg

Mendocino

MENDOCINO COUNTY

Anchor Bay

SONOMA

COUNTY

Stewarts Point

Fort Ross

25 km

Westport

Fort Bragg

MENDOCINO

COUNTY

Mendocino

Anchor Bay

SONOMA

COUNTY

Fort Ross

25 km

Each circle is one area where kelp was detected and the circle's size denotes the square meters of kelp in that area.

In 2008, there were more than 3.5 million square meters of kelp detected at over 8,000 locations along the coastline.

In 2014 and 2015, a marine heat wave severely damaged the kelp forests, just as wildfires above ground hurt the state’s redwood forests. Only 143,000 square meters of kelp were detected at 334 locations.

The kelp forest has since continued to decline to a paltry 11,000 square meters at only 20 locations in 2019.

Warming waters harm kelp forests because they create favorable conditions for purple sea urchins, who feed on them. Their populations have been growing unchecked since the sea star population that feeds on urchins has dropped as a result of sea star wasting disease. Urchins are also resilient and can exist for a long time in starvation mode.

McPherson said some efforts to restore the forest have included removing or limiting urchin populations. Some businesses have even begun harvesting the sea urchins for uni, often served at Japanese sushi restaurants. There have also been efforts to plant kelp to generate new forests, surround existing kelp patches with urchin proof barriers and create a kelp spore bank to preserve the organism’s gene pool, McPherson said.

The kelp forest could be naturally restored as well, if ocean conditions shift to become more favorable for kelp or if a disease kills off purple sea urchins. But without those, urchin barrens — vast areas covered only by urchins — can persist for decades, as has occurred in some areas of Norway.

As the kelp forest dies, so do the creatures that depend on it. Kelp is a key food source for red abalone, which has cultural and economic significance for Mendocino and Sonoma Counties. Without kelp forests, their populations have dropped so much as a result that recreational divers are not allowed to fish for red abalone until at least 2026.

If the forest continues to wither, it will send shockwaves through the marine ecosystem.

"The value of these ecosystems is hard to quantify, but even if humans didn't care or depend on it, the loss of biodiversity, habitat structure, carbon/nutrient cycling is huge," McPherson said.

Data and code are available on GitHub.